This invention is directed to grade measurement devices, commonly referred to as surveying or levelling rods, and is more particularly directed to such a rod which is not only easier to use and to read but which is also characterized by improved versatility while maintaining a relatively simple rod construction. Surveying or levelling rods, hereinafter referred to as "grade" rods, have been widely used in one form or another for hundreds of years. A typical use of such a rod would involve the setting of a rod on a reference point and equating the reading taken to the known elevation of the reference point. When the rod is moved to other locations, a change in elevation would result in a different position of the rod being sighted by the telescope, and in this way the elevations of these other locations can be determined relative to the initial reference elevation.
One problem with such grade rods has been the limited range of elevations which can be measured with any particular rod without recalibrating to a new reference elevation. For example, if a rod is six feet in length and the telescope is set such that it will read from the center of the rod when the rod is at the reference elevation, the rod can only indicate grade variations of .+-.3 feet from the reference elevation. In an application involving a wide range of grade variations, a very long grade rod would be necessary, but this would be unwieldy. Alternatively, repeated instrument setups and recalibrations are necessary.
Some attempts have been made to provide extendable rods, and one technique is to provide extendable calibrated rod sections. However, this results in a somewhat expensive rod in that each extension will require calibrated gradation markings. Some of the previous attempts at providing extendable rods have also provided changeable number sequences. However, the number sequence changing mechanisms have been such that it has been impractical to display numbers at intervals of less than once per foot of rod length. This makes such a rod difficult to use, since the field of view of the sighting telescope may be substantially smaller than one foot. These sequence changing mechanisms have also been impractical for implementing a rod with an effective length of much greater than six feet.
A further disadvantage of most rods is that the number sequences typically increase from bottom-to-top and therefore do not provide a direct reading of the elevation or the elevation change. Since the user must perform mathematical calculations in order to determine the grade variation, there is an increased opportunity for human error.
Still further, even if the number sequence is provided such that it will increase from top-to-bottom of the rod, any given rod includes a number sequence which increases or decreases in only one direction. There are a number of different grade rod applications which will require different types of number sequences, and it has been necessary to use a different rod in each case.